SWIFTX-RAY AND ULTRAVIOLET MONITORING OF THE CLASSICAL NOVA V458 VUL (NOVA VUL 2007)

Abstract

We describe the highly variable X-ray and UV emission of V458 Vul (Nova Vul 2007), observed by Swift between 1 and 422 days after outburst. Initially bright only in the UV, V458 Vul became a variable hard X-ray source due to optically thin thermal emission at kT = 0.64 keV with an X-ray band unabsorbed luminosity of 2.3 × 10 34 erg s −1 during days 71-140. The X-ray spectrum at this time requires a low Fe abundance (0.2 +0.3 −0.1 solar), consistent with a Suzaku measurement around the same time. On day 315 we find a new X-ray spectral component which can be described by a blackbody with temperature of kT = 23 +9 −5 eV, while the previous hard X-ray component has declined by a factor of 3.8. The spectrum of this soft X-ray component resembles those typically seen in the class of supersoft sources (SSS) which suggests that the nova ejecta were starting to clear and/or that the WD photosphere is shrinking to the point at which its thermal emission reaches into the X-ray band. We find a high degree of variability in the soft component with a flare rising by an order of magnitude in count rate in 0.2 days. In the following observations on days 342.4-383.6, the soft component was not seen, only to emerge again on day 397. The hard component continued to evolve, and we found an anticorrelation between the hard X-ray emission and the UV emission, yielding a Spearman rank probability of 97%. After day 397, the hard component was still present, was variable, and continued to fade at an extremely slow rate but could not be analysed owing to pile up contamination from the bright SSS component.

A remarkable recurrent nova in M 31: The X-ray observations

“…2b) might point towards intrinsic variations in the WD temperature. Schwarz et al (2011) suggested a similar scenario to explain the anti-correlated X-ray and UV variability during the SSS phase of the Galactic nova V458 Vul (see also Ness et al 2009). They drew an interesting parallel to the variability in persistent SSSs which might originate in accretion-rate variations that cause the WD photosphere to expand or shrink (see Reinsch et al 2000;Greiner & Di Stefano 2002).…”

Multifrequency nature of the 0.75 mHz feature in the X-ray light curves of the nova V4743 Sgr

“…in V458 Vul (see fig. 6 in Ness et al 2009). Since these novae did not show any persistent oscillations as in V4743 Sgr, our findings are a unique contribution to the discussion about the origin of the early variations in the SSS phase.…”

Obscuration effects in super-soft-source X-ray spectra

“…Before the start of the SSS phase in several novae, emission line spectra had frequently been observed with CCD spectrometers. While their spectral resolution is not high enough to resolve emission lines, the nature as emission line spectra has been established by means of collisional equilibrium models that reproduce these spectra (e.g., Krautter et al 1996;Balman et al 1998;Mukai & Ishida 2001;Orio et al 2001;Ness et al 2009a). Distinct differences to SSe spectra are:…”

“…(3) The emission lines in pre-SSS spectra are in most cases too faint for detection in grating spectra; exceptions are V959 Mon (Ness et al 2012b) and symbiotic novae such as RS Oph (Ness et al 2009b). (4) The early hard X-ray component fades with time (Ness et al 2009a), leaving even less potential contributions to SSe spectra.…”